Automatically switching power supply sources for a clock circuit

ABSTRACT

In one embodiment, a power supply switching circuit may automatically provide power to a clock circuit from one of an auxiliary power supply and a main power supply, based on a voltage of the main power supply. To provide automatic switching, a switch circuit coupled between the power supplies and the clock circuit may be controlled by a voltage detector, in some embodiments.

FIELD OF THE INVENTION

The present invention relates to integrated circuits (ICs), and moreparticularly to clock circuits.

BACKGROUND

Clock circuits are used in many analog ICs to provide reference clocksignals. One particular type of clock circuit is a real time clock(RTC). An RTC is used to provide a reference clock, and can also be usedto provide real time clock functions (i.e., date and time functions). AnRTC typically includes an analog portion and a digital portion. Theanalog portion typically includes an oscillator and buffer circuitry,while the digital portion typically includes digital counters forimplementing the real time functions.

Typically, the oscillator of the analog portion of a real time clock isa crystal oscillator that uses load capacitors to generate a desiredoscillation frequency. The value of the load capacitors may vary,depending on a crystal chosen for use in a system. Accordingly, loadcapacitors are typically implemented using off-chip capacitors,increasing component counts, as well as consuming valuable board area.

Because an RTC is used to provide real time clock functions, it mustalways be supplied power, even when remaining portions of the IC (andeven the system in which it is contained) is powered off. Accordingly,an RTC needs to operate at very low power consumption levels and ittypically runs on a backup power supply, such as a coin cell battery orthe like. To conserve power in this backup power supply, some systemsswitch to provide power to the RTC from a main power supply whenavailable. However, such switching schemes are complex and inefficient,and consume additional power.

A need thus exists to provide more flexibility in the use of loadcapacitors, as well as to reduce power consumed by a back up powersupply.

SUMMARY OF THE INVENTION

In one embodiment, the present invention includes an apparatus that maybe used to automatically switch one of multiple power supplies into acircuit. More specifically, a clock circuit, for example, a real timeclock, can be coupled to receive power from either a primary powersupply or an auxiliary power supply. The apparatus may include a voltagedetector to detect the primary power supply voltage and to generate acontrol signal based on the voltage. The apparatus may further include aswitch circuit coupled to receive the control signal and to coupleeither of the primary power supply or the auxiliary power supply to theclock circuit. In one embodiment, the voltage detector may detect anundervoltage condition of the primary power supply, which may cause theswitch circuit to couple the auxiliary power supply to the clockcircuit.

Yet other embodiments may be implemented in a method for providing powerto circuitry, such as a clock circuit. The method may detect a voltageof a first power supply of a system. Based on the voltage, a selectsignal may be generated. Specifically, the select signal may begenerated with a first state if the voltage of the first power supply issufficient to operate the clock circuit. Thus the first power supply maybe coupled to the clock circuit if the select signal is at the firststate. In contrast, the select signal may be generated with a secondstate if the voltage of the first power supply is insufficient tooperate the clock circuit, and accordingly, a second power supply may becoupled to the clock circuit.

Embodiments of the present invention may be implemented in appropriatehardware, firmware, and software. To that end, one embodiment may beimplemented in an integrated circuit having both analog and digitalcircuitry, including a clock circuit and voltage detection and switchingcircuitry to provide a selected power source to the clock circuit. Stillother embodiments may include a system including such an integratedcircuit along with additional components, such as a host processor,memory, input/output devices and the like.

In one embodiment, the system may be a wireless device such as acellular telephone handset, personal digital assistant (PDA) or othermobile device. Such a system may include a transceiver including a clockcircuit, as well as control circuitry to provide a selected power sourceto the clock circuit. The control circuitry may include a voltagedetector as described above, and switching circuitry to switch anavailable power source to the clock circuit. Of course, the system mayfurther include multiple power sources, including a main power supplyand an auxiliary power supply. The system may further include a hostprocessor coupled to the transceiver.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an analog clock circuit in accordance withone embodiment of the present invention.

FIG. 2 is a block diagram of a clock circuit in accordance with oneembodiment of the present invention that includes the analog clockcircuit of FIG. 1.

FIG. 3 is a block diagram of a portion of an analog clock circuit tocontrol programmable load capacitors in accordance with an embodiment ofthe present invention.

FIG. 4 is a block diagram of a power supply switching circuit inaccordance with one embodiment of the present invention.

FIG. 5 is a block diagram of a system in accordance with one embodimentof the present invention.

DETAILED DESCRIPTION

Referring now to FIG. 1, shown is a block diagram of a clock circuit 10in accordance with one embodiment of the present invention. Morespecifically, clock circuit 10 is an analog portion of a real time clock(RTC) circuit. While described herein as a real time clock circuit, itis to be understood that the scope of the present invention is not solimited and that analog clock circuit 10 may take other forms. Invarious embodiments, analog clock circuit 10 may be implemented in anintegrated circuit (IC) such that virtually all of the components ofanalog clock circuit 10 are on chip, thus reducing cost and board area.

As shown in FIG. 1, analog clock circuit 10 may take the form of acrystal oscillator, such as a Colpitts oscillator although otheroscillator types are possible. Analog clock circuit 10 includes acrystal 20 which may be coupled at nodes V₁ and V₂ to load capacitors C₁and C₂. In various embodiments, load capacitors C₁ and C₂ may be on-chipcapacitors formed as metal-oxide-semiconductor (MOS) capacitors.Furthermore, load capacitors C₁ and C₂ may be programmable such thatanalog clock circuit 10 may be used by various end users havingdifferent crystal requirements. For example, different systemmanufacturers, such as the makers of cellular telephones, personaldigital assistants (PDA) or other such systems, may design in an IC inaccordance with an embodiment of the present invention to operate withdifferent crystal types and/or at different oscillation frequencies. Assuch, load capacitors C₁ and C₂ may be set at different capacitor valuesdepending on the system in which they are installed. As further shown inFIG. 1, analog clock circuit 10 further includes a resistor R coupledbetween nodes V₁ and V₂, along with an amplifier 30.

The oscillator frequency of analog clock circuit 10 may be set usingload capacitors C₁ and C₂. For purposes of extracting a reference signal(which may be a sinusoidal signal) from clock circuit 10, the signal maybe taken either from node V₁ or V₂, or some combination of the voltagesfrom these nodes. In the embodiment shown in FIG. 1, node V₂ is chosento provide the reference signal. Accordingly, in the design shown, thevoltage output at node V₂ may be buffered by buffers 40 and 45 andprovided out of analog clock circuit 10 as a reference clock 50.Reference clock 50 may be used as a system clock directly, or it may befurther processed to obtain one or more different clocks used inoperation of a system.

Crystal 20 may be modeled using an electrical model. More specifically,a given crystal resonator may be modeled as a capacitor and an inductorcoupled in series between the terminals of the crystal. This electricalmodel also includes a capacitor that is connected in parallel to theseries combination of the capacitor and the inductor, and is thusconnected between the terminals of the crystal. Accordingly, thefundamental mode of oscillation of analog clock circuit 10 that includescrystal 20 is that of a series resistance inductance capacitance(RLC)-type oscillator in which a large sinusoidal current I flowsthrough load capacitors C₁ and C₂ and crystal 20. The V₁ voltage isdescribed by the following equation: $\begin{matrix}{V_{1} = {\frac{I}{{sC}_{1}}.}} & \left\lbrack {{Eq}.\quad 1} \right\rbrack\end{matrix}$

The V₂ voltage is described by the following equation: $\begin{matrix}{V_{2} = {\frac{I}{{sC}_{2}}.}} & \left\lbrack {{Eq}.\quad 2} \right\rbrack\end{matrix}$

For embodiments of the invention in which the V₂ voltage is taken as thereference signal, any step change in the capacitance C₂ results in astep change in the V₂ voltage, when it is assumed that the amplitude ofthe I current remains constant. The I current, in fact, remainsrelatively constant in amplitude because the bulk of the I current thatflows through crystal 20 ends up flowing through the inductor in themodel of crystal 20.

Referring now to FIG. 2, shown is a block diagram of a clock circuit 100that includes analog clock circuit 10. More specifically, clock circuit100 may be a real time clock, although the scope of the presentinvention is not so limited. As shown in FIG. 2, analog clock circuit 10provides reference clock 50 that is coupled to a digital portion 110.Digital portion 110 provides control signals 60 to analog clock circuit10 to control the frequency of oscillation.

That is, in embodiments in which load capacitors are implementedon-chip, they may be programmable so that clock circuit 100 may providea desired reference clock using different types of associated crystals.In such manner, different customer's requirements (e.g., with respect tocrystal selection and reference clock frequency) may be met.Furthermore, in embodiments in which clock circuit 100 is an RTC₁ it isto remain in operation even when other blocks of an IC including clockcircuit 100 are not operating. For this reason, in various embodimentsvalues corresponding to control signals 60 may be stored in digitalportion 110. As will be discussed further below, digital portion 100 maystore an initial value for control signals 60 that may then be laterupdated by system software when system software begins operation.

Specifically, registers 115 within digital portion 1 10 may store acontrol value for load capacitors C₁ and C₂ of analog clock circuit 10.As an example, a plurality of registers 115 may be present in digitalportion 110, each of which may store a single bit to be used incontrolling the load capacitors. In one embodiment, registers 115 eachmay be a storage device such as a D-type flip-flop, although the scopeof the present invention is not so limited. Digital portion 110 mayinclude additional registers that perform other functions, such as realtime clock functions. Furthermore, other registers and logic withindigital portion 110 may be used to receive reference clock 50 andprovide the reference clock and/or processed variations of the clock toother circuitry to which clock circuit 100 is coupled.

In one embodiment, three registers 115 may be included in digitalportion 110 and may each provide a single bit value to analog clockcircuit 10 via control signals 60. In turn, control signals 60 may beused in analog clock circuit 10 to control the capacitance of loadcapacitors C₁ and C₂, as will be described further below.

As further shown in FIG. 2, a power on reset circuit 120 is coupled toreceive an operating voltage (i.e., V_(RTC)) and to generate a resetsignal that is provided to digital portion 110. More specifically, thereset signal is provided to registers 115 of digital portion 110, aswill be discussed further below. Also shown in FIG. 2, analog clockcircuit 10 and digital portion 110 are also coupled to receive thesupply voltage, V_(RTC). As will be discussed further below, the supplyvoltage may be provided by a main power supply or an auxiliary powersupply.

In embodiments in which clock circuit 100 is a real time clock, thecircuit 100 is typically the first block on an IC to begin operation andfurthermore it remains in operation while all other blocks on the chipare powered down. Accordingly, the load capacitors (within analogportion 10) may be controlled to have an initial value upon initialpowering of clock circuit 100 (i.e., before a remainder of the IC ispowered on), so that a reference clock can be generated. In variousembodiments, programming the initial capacitor values may be performedupon initial powering by a main power supply. Thus, on initial poweringup of clock circuit 100, registers 115 may be set with an initial valuethat is provided as control signals 60 to analog clock circuit 10 toallow it to begin oscillating and provide a reference clock. When thereference clock is provided to digital portion 110 (and other blocks ofa system), additional system functionality can occur.

In one embodiment, on first powering of clock circuit 100 (i.e., byapplication of V_(RTC)) registers 115 may store initial values to enableinitial programming of load capacitors C₁ and C₂. In one embodiment,power on reset circuit 120 may generate a reset signal that can be usedto set initial register values for registers 115. To provide thisability, power on reset circuit 120 may include delay circuitry suchthat its output is delayed by a short time from its receipt of thesupply voltage (i.e., V_(RTC)). For example, in one embodiment power onreset circuit 120 may include a resistor-capacitor (RC) circuit toprovide a short delay between receipt of the supply voltage and outputof the reset signal. This delay allows the supply voltage to be receivedby digital portion 110, and to power up registers 115. While the timedelay may vary, in some embodiments a delay of between approximately 5microseconds (μs) and 100 μs may be provided.

In one embodiment, registers 115 may store a value of zero upon receiptof the reset signal. These zero values may then be sent via controlsignals 60 to analog clock circuit 10. In such an embodiment, theinitial values provided by registers 115 on control signals 60 may causeload capacitors C₁ and C₂ to be set to a capacitance of betweenapproximately 2.0 and 5.0 picoFarads (pF), although the scope of thepresent invention is not so limited. Furthermore, while a single set ofcontrol signals and decode logic may be used to control both loadcapacitors C₁ and C₂, it is to be understood the scope of the presentinvention is not so limited, and different values (generated bydifferent registers and different decode logic) may be separatelygenerated to select desired capacitances for capacitors C₁ and C₂.

Based on the initial register values it receives, analog clock circuit10 can begin oscillating and provide reference clock 50 to digitalportion 110. Digital portion 110 may then provide the reference clock toother circuitry within a system in which it is included, both on thesame chip, as well as to off-chip components. As an example, thereference clock may be provided to an on-chip baseband processor (notshown in FIG. 2) to enable additional steps in a power on process to beperformed.

As part of such a power on process, which may be implemented usingsoftware such as a basic input/output system (BIOS) or other suchstartup software, the baseband processor may cause registers 115 to bewritten with an operating value that is different from the initialvalue. Thus the operating value may be selected and provided toregisters 115 using software (or a combination of software, firmware andhardware) that may be executed within a system, such as a wirelesssystem or other device using clock circuit 100. The operating value maybe retained by registers 115 so long as a supply voltage from either amain power supply or a backup power supply is active. If power from bothsources is lost, the operating value stored in registers 115 may belost, and upon a next power on by the main power supply, the capacitorprogramming may be repeated to select a desired capacitance. Suchembodiments may include an article in the form of a machine-readablestorage medium onto which there are stored instructions and data thatform a software program to perform such methods.

In an example embodiment, registers 115 may be rewritten with operatingvalues that correspond to a desired capacitance for load capacitors C₁and C₂ for a given system in which the real time clock is to beincluded. In turn, registers 115 may provide the operating values ascontrol signals 60 to enable analog circuit 10 to select the desiredcapacitances, causing the desired oscillation frequency to be attained.In one embodiment, reference clock 50 may be set to operate at 32.768kHz, although the scope of the present invention is not so limited.

Different manners of selecting capacitance levels for analog clockcircuit 10 may be effected in different embodiments. In one embodiment,decode logic and switch devices may be used to select a desiredcapacitance. Referring now to FIG. 3, shown is a block diagram of aportion of analog clock circuit 10. As shown in FIG. 3, a decode logic150 is coupled to a capacitor bank formed of a plurality of capacitorsC₁ to C_(N). While shown in the embodiment of FIG. 3 as including threeindividual capacitors, more or fewer such capacitors may be present in agiven embodiment. As discussed above, in some embodiments the capacitorsmay be MOS capacitors. In other embodiments, the capacitors may bemetal-insulator-metal (MIM) capacitors or metal finger capacitors.Decode logic 150 receives the incoming control signals 60 from digitalportion 110. Based on the values of the incoming control signals 60,decode logic 150 generates switching signals to control switching ofswitching devices 165 to selectively couple associated capacitorsC₁-C_(N) of the capacitor bank between a capacitor array line 170 andground. While not shown in FIG. 3, it is to be understood that line 170may be coupled to nodes V₁ or V₂ of FIG. 1. The oscillator's frequencymay be controlled by the capacitor bank, and more specifically byselectively coupling to and isolating certain of the capacitors from aresonant tank of the oscillator in response to control signals 60. Thatis, selected ones of capacitors C₁-C_(N) of the capacitor bank arecoupled by capacitor array line 170 to the oscillator core. Thecapacitance that appears on line 170 controls the oscillation frequencyof the oscillator core and therefore, controls the oscillation frequencyof reference clock signal 50 (of FIG. 1).

For purposes of controlling the level of capacitance that appears online 170, decode logic 150 and switching devices 165 selectivelyestablish connections between the capacitors of the capacitor bank andground. As shown in FIG. 3, switching devices 165 may be n-channelmetal-oxide-semiconductor field-effect transistors (nMOSFETs), althoughother configurations are possible. Each nMOSFET 165 has a drain terminalcoupled to a terminal of an associated capacitor. A source terminal iscoupled to a ground potential, and each nMOSFET 165 has a gate terminalcoupled to receive a selection signal from decode logic 150.

One terminal of each capacitor C₁-C_(N) is coupled to line 170, whichmay be coupled to node V₁ or V₂ of FIG. 1. In response to controlsignals 60, decode logic 150 and switching devices 165 selectivelycouple the capacitors C₁-C_(N) to ground so that when a particularcapacitor is coupled to ground, the capacitor becomes coupled to theoscillator and contributes to the capacitance of line 170. Otherwise,the capacitor remains isolated (i.e., “free floating”) from theoscillator and does not contribute to the capacitance of line 170.

While shown with a particular configuration in the embodiment of FIG. 3,both decode logic 150 and switching devices 165 may take on variousforms, depending on the particular embodiment. For example, decode logic150 may be a thermometer-based decode logic that, in response to controlsignals 60 generates select signals to selectively control theactivation of nMOSFETs 165, which selectively couple associatedcapacitors C₁-C_(N) between line 170 and ground. In such an embodiment,the gate terminal of each nMOSFET 165 receives a binary selection signalfrom decode logic 150. Thus, the activation (via its gate terminal) of aparticular nMOSFET 165 connects the associated capacitor to line 170,adding capacitance to line 170. This additional capacitance, in turn,changes the oscillation frequency. Likewise, the de-activation (turningoff, for example) of a particular nMOSFET 165 (via its gate terminal)removes capacitance from line 170 and thus, affects the oscillationfrequency in the opposite direction.

In some embodiments of the invention, capacitors C₁-C_(N) may each havethe same unit capacitance. In other words, decode logic 150 determines,based on the incoming control signals 60, how many of the capacitors areto be coupled to line 170 and activates the appropriate number ofnMOSFETs 165. Of course, many other embodiments are possible and arewithin the scope of the appended claims. For example, instead of havinga decode logic, switching devices (formed of nMOSFETs or other suchdevices) may directly receive a particular bit of control signals 60(e.g., via a gate terminal of the MOSFET).

Furthermore, instead of each being associated with a capacitor of thesame unit size, each switching device may selectively couple abinary-weighted capacitor to ground. Due to such binary weighting, themore significant bits of control signals 60 control the coupling of themore significant capacitance to line 170.

While FIG. 3 shows a single capacitance array line 170, it is to beunderstood that mirrored capacitor banks may be present, one of which iscoupled to node V₁ (of FIG. 1) and the other of which is coupled to nodeV₂ (also of FIG. 1). In these embodiments of the invention, changes tothe capacitance C₁ may occur concurrently with changes to thecapacitance C₂.

As discussed above, the supply voltage for a clock circuit may beprovided by a main power supply or an auxiliary power supply. In anembodiment used in a wireless device, such as a cellular telephone,personal digital assistant (PDA), portable computer, or the like, themain power supply may be AC power or a battery pack, and the auxiliarypower supply may be a small coin cell type battery, other batterysupply, or large capacitor, to power the real time clock when theremainder of the system is powered down.

In various embodiments, power to a real time clock circuit may beautomatically switched between a main power supply and an auxiliarypower supply, based upon the state of the main power supply. That is,the real time clock may be powered by the main power supply when it isavailable, and instead be powered by the auxiliary power supply when themain power supply is unavailable.

In one embodiment, switching circuitry may be provided to automaticallyswitch the power supplies into or out of the real time clock circuit.Referring now to FIG. 4, shown is a block diagram of a switch circuit inaccordance with one embodiment of the present invention. As shown inFIG. 4, a power supply switching circuit 200 includes a switch complex210 that is coupled to receive both a main power supply voltage(V_(Main)) and an auxiliary power supply voltage (V_(Backup)), andprovide a selected one of the power supply voltages as the supplyvoltage for the real time clock (V_(RTC)) via an output node 235. In oneembodiment, V_(Main) may be between approximately 2.7 volts and 3.0volts, while V_(Backup) may be between approximately 2.4 to 3.3 volts.

Power supply switching circuit 200 further includes an undervoltagelockout block (UVLOB) circuit 220 that is coupled to receive the mainpower supply voltage and to generate an output signal based on the levelof the main power supply voltage. That is, UVLOB circuit 220 includes avoltage detector to detect the voltage of the main power supply andprovide a control signal based on its value. More specifically, UVLOBcircuit 220 generates a control signal 225 that is coupled to aninverter 230 that in turn is coupled to a node 219 of switch complex210. Control signal 225 is further coupled to a node 215 of switchcomplex 210.

Still referring to FIG. 4, switch complex 210 includes a first pair oftransistors coupled to the auxiliary power supply voltage. Specifically,a first p-channel MOSFET (pMOSFET) 212 has a source terminal coupled tothe auxiliary power supply and a drain terminal coupled to a drainterminal of a second pMOSFET 214, which has a source terminal coupled tooutput node 235 that provides the selected power supply voltage to thereal time clock. The transistor pair including pMOSFETs 212 and 214 iscontrolled by control signal 225, which is coupled to node 215 that iscoupled to the gate terminals of both pMOSFETs 212 and 214.

Similarly, switch complex 210 further includes a second pair oftransistors coupled to the primary power supply voltage. Specifically, athird p-channel MOSFET (pMOSFET) 216 has a source terminal coupled tothe primary power supply and a drain terminal coupled to a drainterminal of a fourth pMOSFET 218, which has a source terminal coupled tooutput node 235 that provides the selected power supply voltage to thereal time clock. The transistor pair including pMOSFETs 216 and 218 iscontrolled by the inverted value of control signal 225, which is coupledto node 219 that is coupled to the gate terminals of both pMOSFETs 216and 218. While shown with a particular configuration in FIG. 4, it is tobe understood that switch complex 210 may be configured differently inother embodiments. For example, in some embodiments nMOSFETs may beused. In other embodiments an active current mirror or a form of gainstage may be implemented.

UVLOB circuit 220 is used to measure the voltage of the main powersupply. When the voltage is greater than a predetermined threshold,which may be equal to the lowest voltage at which an associated realtime clock can operate, UVLOB circuit 220 generates a logic high signalas control signal 225. In one embodiment, the threshold voltage may beapproximately 2.4 volts. When control signal 225 is high, pMOSFETs 212and 214 are gated off and pMOSFETs 216 and 218 are gated on, allowingthe main power supply voltage to be coupled to node 235 and provided tothe real time clock. In contrast, when the main power supply voltage isbelow the selected threshold, UVLOB circuit 220 generates a logic lowvalue as control signal 225, switching on pMOSFETs 212 and 214 andturning off pMOSFETs 216 and 218. Accordingly, the auxiliary powersupply voltage is coupled to node 235 to operate the real time clock.With the configuration shown in FIG. 4, switch complex 210 can couplethe auxiliary power supply to node 235, even when the main power supplyis powered off. Thus in systems in which switching circuit 200 iscoupled to provide power to a real time clock circuit, such power can beprovided even when a remaining portion of the system is powered off.

In such manner, switching circuit 200 can automatically switch the realtime clock to use either a main power supply or an auxiliary powersupply. This may conserve the auxiliary power supply, in someembodiments. While described as being connected to an RTC circuit, it isto be understood that a power supply switch in accordance with anembodiment of the present invention can be used to switch between powersupplies for any type of circuitry.

Referring now to FIG. 5, shown is a block diagram of a system inaccordance with one embodiment of the present invention. As shown inFIG. 5, system 300 may be a wireless device, such as a cellulartelephone, PDA, portable computer or the like. An antenna 305 is presentto receive and transmit radio frequency (RF) signals. Antenna 305 mayreceive different bands of incoming RF signals using an antenna switch.For example, a quad-band receiver may be adapted to receive globalsystem for mobile (GSM) communications, enhanced GSM (EGSM), digitalcellular system (DCS) and personal communication system (PCS) signals,although the scope of the present invention is not so limited. In otherembodiments, antenna 305 may be adapted for use in a general packetradio service (GPRS) device, a satellite tuner, or a wireless local areanetwork (WLAN) device, for example.

Incoming RF signals are provided to a transceiver 310 which may be asingle chip transceiver including both RF components and basebandcomponents. Transceiver 310 may be formed using a complementarymetal-oxide-semiconductor (CMOS) process, in some embodiments. Thus asshown in FIG. 5, transceiver 310 includes an RF transceiver portion 312and a baseband processor 314. RF transceiver portion 312 may includereceive and transmit portions and may be adapted to provide frequencyconversion between the RF spectrum and a baseband. Baseband signals arethen provided to a baseband processor 314 for further processing.

As further shown in FIG. 5, transceiver 310 further includes a real timeclock 316 that is used to provide a reference clock signal to basebandprocessor 314 based on control signals received from baseband processor314. Furthermore, while not shown in FIG. 5, it is to be understood thatthe reference clock signal (or processed variations thereof) may beprovided to other circuitry within system 300. RTC 316 may includeon-chip load capacitors C₁ and C₂ which, as described above, may beprogrammable so that the desired reference clock signal may be generatedusing any one of a number of different crystals. While shown in theembodiment of FIG. 5 as being including within transceiver 310, in otherembodiments RTC 316 may be located off chip. In yet other embodiments,RTC 316 may be on chip, while load capacitors C₁ and C₂ may be locatedoff chip. In the embodiment of FIG. 5, a crystal 315 is coupled to RTC316.

Still referring to FIG. 5, transceiver 310 may further include a powersupply (PS) switching circuit 318. PS switching circuit 318, which maybe formed of switching circuit 200 of FIG. 4, may provide a selected oneof a main power supply and an auxiliary power supply to RTC 316.Further, while not shown in FIG. 5, it is to be understood that PSswitching circuit 318 may further provide the selected supply voltage toother portions of transceiver 310, in addition to other parts of system300. While shown in FIG. 5 as being included within transceiver 310, itis to be understood that the scope of the present invention is not solimited, and in other embodiments PS switching circuit 318 may be offchip.

After processing signals received from RF transceiver 312, basebandprocessor 314 may provide such signals to various locations withinsystem 300 including, for example, an application processor 320 and amemory 330. Application processor 320 may be a microprocessor, such as acentral processing unit (CPU) to control operation of system 300 andfurther handle processing of application programs, such as personalinformation management (PIM) programs, email programs, downloaded games,and the like. Memory 330 may include different memory components, suchas a flash memory and a read only memory (ROM), although the scope ofthe present invention is not so limited. Additionally, a display 340 isshown coupled to application processor 320 to provide display ofinformation associated with telephone calls and application programs,for example. Furthermore, a keypad 350 may be present in system 300 toreceive user input.

Although the description makes reference to specific components ofsystem 300, it is contemplated that numerous modification and variationsof the described and illustrated embodiments may be possible.Furthermore, while not shown in FIG. 5, it is to be understood that aprimary and auxiliary power supply may be present, both of which may becoupled to power supply switching circuit 318 in accordance with anembodiment of the present invention to automatically switch a supplyvoltage to RTC 316, based on a voltage level of the primary powersupply.

While the present invention has been described with respect to a limitednumber of embodiments, those skilled in the art will appreciate numerousmodifications and variations therefrom. It is intended that the appendedclaims cover all such modifications and variations as fall within thetrue spirit and scope of this present invention.

1. An apparatus comprising: a voltage detector to detect a voltage of afirst power supply and to generate a control signal based on the voltageof the first power supply; and a switch circuit coupled to receive thecontrol signal and to couple either of the first power supply or asecond power supply to a clock circuit.
 2. The apparatus of claim 1,wherein the clock circuit comprises a real time clock.
 3. The apparatusof claim 1, wherein the switch circuit is to automatically switchbetween the first power supply and the second power supply based on thecontrol signal.
 4. The apparatus of claim 3, wherein the switch circuitcomprises a first transistor pair controllable via the control signaland a second transistor pair controllable via an inversion of thecontrol signal.
 5. The apparatus of claim 4, wherein the firsttransistor pair is coupled between the second power supply and the clockcircuit.
 6. The apparatus of claim 1, wherein the voltage detector is togenerate the control signal when the voltage of the first power supplyexceeds a threshold voltage, the threshold voltage comprising a minimumvalue to power the clock circuit.
 7. The apparatus of claim 1, whereinthe switch circuit is to couple the second power supply to the clockcircuit when the first power supply is off.
 8. The apparatus of claim 1,wherein the switch circuit is to couple the second power supply to theclock circuit when a remaining portion of the apparatus including thefirst power supply is off.
 9. A method comprising: detecting a voltageof a first power supply of a system; generating a select signal with afirst state if the voltage of the first power supply is sufficient tooperate a clock circuit; and coupling the first power supply to theclock circuit if the select signal is at the first state.
 10. The methodof claim 9, further comprising generating the select signal with asecond state if the voltage of the first power supply is insufficient tooperate the clock circuit.
 11. The method of claim 10, furthercomprising coupling a second power supply to the clock circuit if theselect signal is at the second state.
 12. The method of claim 11,further comprising coupling the select signal to a first transistor paircoupled between the first power supply and the clock circuit and asecond transistor pair coupled between the second power supply and theclock circuit.
 13. The method of claim 11, wherein the second powersupply comprises an auxiliary power supply, and the first power supplycomprises a main power supply that is switched off when the system ispowered off.
 14. A wireless system comprising: a clock circuit; avoltage detector to detect a voltage level of a primary power supply andto generate a control signal; a selection circuit coupled to receive thecontrol signal and to couple either the primary power supply or anauxiliary power supply to the clock circuit, based upon the controlsignal; and an antenna coupled to the clock circuit.
 15. The wirelesssystem of claim 14, further comprising a transceiver including thevoltage detector and the selection circuit.
 16. The wireless system ofclaim 14, wherein the selection circuit is to automatically switchbetween the primary power supply and the auxiliary power supply based onthe control signal.
 17. The wireless system of claim 14, wherein theselection circuit comprises a first transistor pair controllable via thecontrol signal and a second transistor pair controllable via aninversion of the control signal.
 18. The wireless system of claim 17,wherein the first transistor pair is coupled between the auxiliary powersupply and the clock circuit.
 19. The wireless system of claim 15,wherein the transceiver further comprises a baseband processor.
 20. Thewireless system of claim 14, wherein the primary power supply isswitched off when the wireless system is powered off.